Most large concrete structures include a skeleton of welded steel rods for reinforcement. Because concrete is permeable by water, the steel rods eventually rust and corrode. The problem of corrosion of steel reinforcement is extreme in the case of a concrete column or similar structure that is partially submerged in seawater, such as a bridge piling; the salt ions aid corrosion and partial immersion in water helps drive electrochemical reactions, which are generally deleterious. Another significant source of corrosion of steel reinforcement is de-icing salt, which especially affects the deck of a bridge.
Corrosion of the steel is harmful to the structure. As the steel rods are dissolved or replaced by rust, they lose strength. Rust stains on the structure are ugly and may cause worry in persons using the structure. Corroded Steel has a greater volume than uncorroded steel; this expansion can crack the concrete and cause chunks to spall. Corrosion of the steel reinforcement can lead to eventual failure of the structure.
A widely used method of repairing cracked and spalled concrete structures, including bridge pilings, is to wrap structural elements in high-strength fiber-reinforced polymer composite panels. The wrap strengthens the structural element and partially shields it from further infiltration by water. A small amount of expansion of the steel due to residual corrosion slightly strengthens the composite wrap by putting it in tension. This method is discussed in more detail in U.S. Pat. No. 5,607,527, incorporated herein by reference.
In the case of structures in very corrosive environments, such as partly submerged in seawater, the composite wrap method does not protect the structure for as many years as is usually desired. Therefore, there is a need for a repair and protection method that has the many advantages of the composite wrap method, but that provides a longer reliable lifetime for structures in very corrosive environments.